Various kinds of structure for electric position adjustment apparatuses for a steering wheel are known, and some are already being applied to steering apparatuses for automobiles. FIG. 21 illustrates a first example of conventional structure as disclosed in JP 2010-116042 (A). A steering shaft 2 to which a steering wheel 1 is fastened has a cylindrical shaped outer shaft 3, and a circular rod shaped inner shaft 4 that is inserted into the outer shaft 3 and is able to slide in the axial direction and transmit rotational force. More specifically, the inner circumferential surface from the middle section to the front-end section of the outer shaft 3 and the outer circumferential surface of the rear-end section of the inner shaft 4 are connected with spline engagement. The steering wheel 1 is fastened to the rear-end section of the outer shaft 3.
The steering shaft 2 is inserted into a cylindrical steering column 5, and is supported so as to be able to rotate freely. The steering column 5 has a cylindrical shaped outer column 6 that is supported by the vehicle body, and a cylindrical inner column 7 that is inserted into the outer column 6 so as to be able to slide in the axial direction. The rear end side portion of the middle section of the outer shaft 3 is supported on the inner side of the rear-end section of the inner column 7 such that displacement in the axial direction is prevented, and so as to be able to rotate freely. Moreover, the front end side portion of the middle section of the inner shaft 4 is supported on the inner side of the front-end section of the outer column 6 such that displacement in the axial direction is prevented, and so as to be able to rotate freely. With this structure, the steering shaft 2 is supported on the inside of the steering column 5 so as to be able to rotate freely, and the outer shaft 3 and inner column 7 are able to move in the forward/backward direction relative to the inner shaft 4 and outer column 6.
The apparatus of this first example of conventional structure is equipped with an electric actuator for making it possible to adjust the forward/backward position of the steering wheel 1 of a gear housing 8 that is fastened to the bottom surface of the outer column 6; a feed nut 9 that is supported on the inside of the gear housing 8 so that only rotation is possible with displacement in the axial direction prevented; a push-pull arm 10 that is fastened to the portion on the rear-end section of the inner column 7 that protrudes further toward the rear than the outer column 6; a push-pull rod 11 having a male screw section 12 provided on the front half section thereof that screws into the feed nut 9, and a rear-end section that is connected to the push-pull arm 10; and an electric motor (not illustrated in the figure) that is connected to the feed nut 9 by way of a worm reduction gear 13, and that rotates and drives the feed nut 9.
When adjusting the forward/backward position of the steering wheel 1, the feed nut 9 is rotated and the push-pull rod 11 is caused to displace in the axial direction. As the push-pull rod 11 displaces, the inner column 7 displaces in the same direction as the push-pull rod 11 by way of the push-pull arm 10, and by causing the outer shaft 3 that is supported on the inside of the inner column 7 to move in the forward/backward direction, the forward/backward position of the steering wheel 1 is adjusted.
In the electric position adjustment apparatus for a steering wheel of this first example of conventional construction, there is a possibility that the driver that is operating the steering wheel 1 will have a strange or uncomfortable feeling due to a small gap that exists in a joint section between the rear-end section of the outer column 6 and the front-end section of the inner column 7. In other words, in an electric position adjustment apparatus for a steering wheel, differing from a manual apparatus, the diameter of the rear-end section of the outer column 6 is not reduced even when maintaining the position of the steering wheel 1 after adjustment, so the small gap remains in the joint section. There is a possibility that the inner column 7 that supports the outer shaft 3 to which the steering wheel 1 is fastened may move or vibrate with respect to the outer column 6 that is supported by the vehicle body. There is also a possibility, that due to this movement, the support rigidity of the steering wheel 1 will decrease, giving the driver that operates the steering wheel 1 an uncomfortable feeling. Moreover, this backlash movement causes the resonant frequency of the portion of the electric position adjustment apparatus for a steering wheel that makes up the steering column apparatus that includes the steering shaft 2 and steering column 5 to become lower, and thus when there is minute vibration of the vehicle body such as when traveling over a rough road, there is a possibility that unpleasant noise or vibration will occur in this portion of the steering column apparatus.
FIG. 22 and FIG. 23 illustrate a second example of conventional structure as disclosed in JP 2006-297989 (A). In this second example of conventional structure as well, using construction similar to that of the first example of conventional structure, a steering shaft 2a is supported inside a steering column 5a so as to be able to rotate freely, and an outer shaft 3a and inner column 7a move in the forward and backward direction relative to an inner shaft 4a and outer column 6a. 
In the apparatus of this second example of conventional structure, by using a linear motion ultrasonic motor 15 that is supported by the bottom end section of an installation bracket 14 as an electric actuator for making it possible to adjust the forward/backward position of the steering wheel 1, it is possible for the inner column 7a to displace in the axial direction with respect to the outer column 6a. More specifically, a moving piece 16 of the linear motion ultrasonic motor 15 is connected with the inner column 7a by a transmission member 17, and by transmitting the movement of the moving piece 16 to the inner column 7a, the inner column 7a is able to move in the forward/backward direction. The base end section of the transmission member 17 is linked with the moving piece 16 by way of a spherical joint 18. The spherical joint 18 has: a concave engaging section 19 that is formed in the moving piece 16; a spacer 20 that fits with and is supported by the concave engaging section 19, and that has an inner circumferential surface that is a spherical concave surface; a spherical engaging section 21 that is provided on the tip-end section of the transmission member 17, and has an outer circumferential surface that is a spherical convex surface that fits spherically with the inner circumferential surface of the spacer 20.
When adjusting the forward/backward position of the steering wheel 1, the linear motion ultrasonic motor 15 causes the moving piece 16 to displace in the axial direction of the steering column 5a. This displacement, by way of the transmission member 17, causes the inner column 7a to displace in the same direction as the moving piece 16, which causes the outer shaft 3a that is supported on the inside of the inner column 7a to displace in the forward/backward direction together with the inner column 7a. As a result, as illustrated by the two-dot chain line, it becomes possible to adjust the forward/backward position of the steering wheel 1.
In the case of this second example of conventional structure, there is a structural problem in that the manufacturing cost for maintaining smooth operation of the engaging section between the moving piece 16 and the transmission member 17 increases. In other words, in order to smoothly perform adjustment of the forward/backward position of the steering wheel 1 without backlash movement, it is necessary to adequately regulate the engagement strength of the engaging section between the spherical engaging section 21 of the transmission member 17 and the engaging section of the spacer 20. When this engagement strength is too low, there is a positive gap in the engaging section, so backlash movement occurs between the spherical engaging section 21 and spacer 20, and thus it becomes easy for backlash movement of the steering wheel 1 to occur in the forward/backward direction. On the other hand, when this engagement strength is too high, it becomes impossible for pivotal displacement of the spacer 20 with respect to the spherical engaging section 21 to be performed smoothly.
In this second example of conventional structure, the amount of movement of the moving piece 16 in the forward/backward direction is large, and unless there is a good degree of parallelism between the direction of movement of this moving piece 16 and the direction of movement of the inner column 7a during adjustment of the forward/backward position of the steering wheel 1, the transmission member 17 will displace in the axial direction of this transmission member 17 relative to the moving piece 16 due to the adjustment of the forward/backward position of the steering wheel 1. This relative movement causes rubbing in the axial direction of the transmission member 17 between the outer circumferential surface of the spacer 20 and the inner circumferential surface of the concave engaging section 19. When this occurs and the engagement strength is too high and pivotal displacement of the spacer 20 with respect to the spherical engaging section 21 is not performed smoothly, there will be strong rubbing between the outer circumferential surface of the spacer 20 and the inner circumferential surface of the concave engaging section 19, and there will be a possibility that strange noise or vibration will occur, causing the passengers to have an uncomfortable feeling. Particularly, in this state, the surface that allows relative displacement in the axial direction of the transmission member 17 between the transmission member 17 and the moving piece 16 is limited to one location on the outer circumferential surface of the spacer 20 and the inner circumferential surface of the concave engaging section 19, so it becomes easy for the rubbing length to become long, and thus it becomes even easier for strange noise or vibration to occur.
Highly precise processing of the inner circumferential surface of the spacer 20, which is a concave spherical surface, and the outer circumferential surface of the spherical engaging section 21, which is a convex spherical surface, in order to prevent the occurrence of this strange noise or vibration leads to a high cost. Moreover, even when the spherical engaging section 21 is accurately manufactured, it is impossible to avoid the outer diameter of the transmission member 17 from becoming small at the base end section of the spherical engaging section 21. Therefore it becomes easy for large stresses to occur in the neck section of the transmission member 17 during adjustment of forward/backward position of the steering wheel 1, and thus this kind of construction is disadvantageous from the aspect of maintaining sufficient durability during use over a long period of time.
As a structure for preventing the occurrence of strange noise or vibration in portions of the steering column apparatus, there is a mechanism such as illustrated in FIG. 24 and disclosed in JP 6,076,908 (B2) in which retaining holes 22 are formed in part in the axial direction of an outer column 6b, and by adjustment screws 24 that are screwed into female screws 23 that are formed in the retaining holes 22 pressing synthetic resin pads 26 so as to be in contact with the outer circumferential surface of an inner column 7b, backlash movement of portions of the steering column apparatus is prevented. However, in this mechanism, when the pressure force for pressing the pads 26 against the outer circumferential surface of the inner column 7b is made just large enough so that backlash movement can be prevented, the friction force when moving the outer column 6b and inner column 7b becomes large, so there is a problem in that the drive load on the motor, such as an electric motor, becomes large and the operating sound becomes large.